A nuclear power plant differs from a conventional power plant by the way is generated. While steam is produced in a conventional thermal power plant by means of a combustion process in a steam boiler, in a nuclear power plant, a fission process takes place in a reactor.

The production of steam in a PWR (pressurized water reactor) is achieved in several steps:

1. In the reactor, fuel rods transfer their heat to a primary circuit containing water. This water becomes extremely hot - approximately 300°C - but does not boil as it is kept pressurized at approximately 155 bar. Hence the name "pressurized water reactor".

2. The heated water is routed to a heat exchanger, i.e. the steam generator. The heat from reactor coolant water is transferred to a separate secondary circuit (the water-steam circuit). Due to a lower pressure in this circuit the water is converted into steam; the steam is then used to drive a turbine connected to a generator.

3. Finally, the steam leaving the turbine is converted back into water by cooling it. This cooling process occurs in the condenser via a separate tertiary cooling circuit, using water from an external source.

PWR nuclear power plants therefore have three separate circuits:

• the primary circuit which transfers the heat from the reactor via the steam generator to the secondary circuit

• the secondary circuit (water-steam circuit)

• the tertiary cooling circuit which converts the expanded steam back into water.

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